The present invention relates to a control system for a construction apparatus which includes a tool that is capable of vertical and/or horizontal adjustment and, more particularly, to a control system which includes a device for measuring a reference in order to adjust the height and/or lateral displacement of the tool in response to the location of the reference.
Conventional controls that assist an operator of a construction apparatus in following a reference datum include mechanical sensors and, more recently, electromagnetic and acoustic sensors, for example ultrasonic transducer sensors. Mechanical sensors are mounted on the construction apparatus and typically contact a string which provides a reference datum. The location information of the reference datum is then used as input into a control system which adjusts the elevation or lateral position of the construction apparatus, including for example a tool such as a blade. However, these mechanical sensors often displace the string and, consequently, do not necessarily provide an accurate indication of the desired reference datum. Additionally, mechanical sensors are prone to being displaced off the string and require manual replacement back on the string.
Various control mechanism have been developed for construction implements, for example, some road graders include mechanical controls to adjust the lateral position of the blade relative to the grader. In U.S. Pat. No. 4,733,355 entitled NONCONTACTING RANGE SENSING AND CONTROL DEVICE shows a grader which includes a sensor that measures the lateral distance from a curb. However, the reference does not provide a practical scheme for controlling the blade height and lateral positions with respect to a curb. Further, the reference does not show a practical scheme for following a string.
Electromagnetic and acoustic sensors provide an improvement over mechanical sensors and offer a broader application. For example, electromagnetic and acoustic sensors can be used to locate a curb, a road bed, or the like as the datum which are less susceptible to inaccuracies since these datums do not move. However, these references are subject to anomalies, for example man holes. Furthermore, electromagnetic sensors and acoustic sensors require more elaborate electronic and/or circuitry. In addition, electromagnetic and acoustic sensors are affected by external factors, such as stray reflections and environmental conditions, including, for example, heat.
For example, U.S. Pat. No. 4,924,374 issued to Middleton et al., entitled METHOD FOR AUTOMATIC POSITION CONTROL OF A TOOL, discloses an ultrasonic sensor and control apparatus for a construction implement. The ultrasonic transducer measures the time it takes for an acoustic pulse to travel from the transducer to the reference surface and back to the transducer. Although these sensors provide an improvement over the mechanical sensors, the ultrasonic sensor only measures the time that it takes for a pulse to travel from the sensor to the reference surface and back; accordingly, the ultrasonic sensor only measures the absolute distance to the reference surface and not the true elevation of the implement. Furthermore, if the sensor is laterally shifted from a reference string, for example, there may be a large error introduced into the measurement.
In addition, since ultrasonic sensors are sensitive to environmental conditions, such as temperature and humidity, ultrasonic sensors require calibration to compensate for changes in the velocity of the acoustic pulse resulting from changes in local air conditions. Increases and decreases in velocity cause inaccuracy in the control system because the distance measured is a function of velocity. Various methods have been developed to overcome this problem. For example, in the Middleton patent, mentioned above, the control system includes a thermistor which senses variations in temperature and compensates for variations in temperature automatically. However, this thermistor does not provide an indication of all the air parameters that can affect the velocity of the acoustic pulse, such as humidity.
U.S. Pat. No. 5,327,345, issued to Edward G. Nielsen et al. entitled POSITION CONTROL SYSTEM FOR A CONSTRUCTION IMPLEMENT SUCH AS A ROAD GRADER, discloses a sensor system which utilizes three transducers. Two of the sensors are used to detect vertical and lateral distances of the sensor relative to a reference surface. The third transducer is used to calibrate the system which allows the velocity of acoustic wave to be accurately measured. In so doing, the third sensor accounts for increases and decreases in velocity due to changes in local air conditions. Consequently, the sensor system of the Nielsen patent measures the true vertical distance and true horizontal distance between the sensor and a reference.
Heretofore, ultrasonic reflectors are placed at a sufficient distance from the transducer so as not to return an echo until such time of the oscillation of the transmitting transducer has stopped or a separate transducer was used for calibration. This oscillation and the subsequent required delayed time in sensors further limits the proximity of the sensor unit to a target since the reflector must be spaced from the transducer. For example, in many applications, a reflector must be placed at a distance of eight to ten inches from the ultrasonic transducer, which as described above exposes the reflector to risk of damage. Consequently, the target for the ultrasonic transducer must be at some distance beyond the reflector so that the ultrasonic transducer can distinguish between the echo from the reflector and that from the target.
Additionally, conventional ultrasonic sensor systems require the datum to be positioned between the primary transducers, which result in the ultrasonic system protruding from the equipment since the sensor system must be positioned over the reference datum. In the U.S. and often in Europe it is necessary to locate the construction equipment in close proximity to or adjacent a structure, such as a building. This datum location may result in contact between the sensor, which is typically outboard from the construction equipment and the structure; therefore, the datum location limits the application of ultrasonic sensors in some cases.
Consequently, there is a need for a control system which has broad application and can be implemented with less complicated electronics and/or circuitry and, preferably, in a self-contained unit with a sensor that permits the construction apparatus to operate in close proximity to or adjacent a structure, such as a building or the like.